The present invention relates to a method for forming perforations in articles of bar-like form utilising laser rays. The present invention finds particularly advantageous application in the field of the production of smoking articles, in particular cigarettes, to which the following discussion will make specific reference without any loss of generality thereby.
For the purpose of avoiding any doubt about the interpretation of various terms utilised hereinafter in the following description and in the claims, it is stated that:
the term "generator" or "laser generator" will be understood to refer to an active element such as, for example, a laser gun capable of generating a beam of laser rays;
the term "reflector" or "laser reflector" will be understood to refer to a passive element capable solely of reflecting a beam of laser rays emitted by a "generator";
the term "emitter" or "laser emitter" will be understood to refer to an element which can be active or passive, that is to say an element which may be a "generator" or may be a "reflector";
the term "focusing point" will be understood to refer to the point at which the rays of a beam of parallel laser rays are deviated to converge at a point or focus;
the term "focal length" will be understood as the distance travelled by a beam of laser rays from the "focusing point" to the focus; and
the term "piercing point" will be understood to refer to the point at which a hole is to be formed.
In the field of cigarette manufacture it is known to form so-called "ventilated" cigarettes, that is to say cigarettes provided in the region of the filter with a plurality of holes which permit the smoker to inhale, together with the smoke, a percentage of external air, with the double advantage of diluting the inhaled smoke and reducing its temperature and, therefore, the content of harmful elements.
In the past, ventilated cigarettes have been made utilising needle devices the efficiency of which has become ever reduced with the increase in productive capacity of cigarette making machines. In fact, if associated with relatively high speed machines, the said needle devices wear out in a relatively short time requiring frequent interruptions in the productive cycle.
For the reasons explained above the said needle devices have recently been replaced, when possible by laser ray perforator devices which have been made according to one of two different constructional arrangements. In the first of the two above mentioned known constructional arrangements a beam of pulsed laser rays is emitted by a fixed laser generator and directed onto a cigarette to be pierced. This latter, during the piercing operation, is made to rotate about itself in such a way that the holes formed on it are distributed in a substantially uniform manner along at least one circumference.
In the other of the two above mentioned known constructional arrangements a high power laser generator is utilised which, for piercing each cigarette, emits a single beam of rays which are broken down by fixed reflectors into a plurality of beams equal in number to the number of holes to be made. In this case the holes in each cigarette are made simultaneously without it being necessary to make the cigarette turn about its axis.
Both of the above described known constructional arrangements have significant disadvantages which make them difficult to put into practice. In fact, the first said constructional arrangement necessarily requires that each cigarette is rolled between two facing surfaces in such a way as to turn about its axis. Since the cigarette has immediately previously already been "softened" following a first rolling performed during the operations for connecting a filter, a further rolling for performing the piercing is particularly damaging in that it certainly involves a partial emptying of the cigarette. The second of the two above described constructional arrangements is on the other hand difficult to put into practice for purely economic reasons given the high power and, therefore, the high cost of the laser generator which must be used.